Preparative process for alkaline earth metal, aluminum-containing spinels

ABSTRACT

An improved process for the production of alkaline earth, aluminum-containing spinel compositions, preferably magnesium, aluminum-containing spinel compositions and preferably further comprising at least one additional metal component, comprises combining alkaline earth metal and aluminum components at selected pH conditions to form a precipitate and calcining the precipitate to form a spinel composition. The product spinel composition, preferably with included additional metal components, is particularly suited for use to reduce the amount of sulfur oxides emitted from a catalyst regeneration zone, e.g., a catalytic cracking unit regeneration zone.

BACKGROUND OF THE INVENTION

This invention relates to the improved preparation of alkaline earthmetal, aluminum-containing spinel compositions, particularly for use inthe combusting of solid, sulfur-containing material in a manner toeffect a reduction in the emission of sulfur oxides to the atmosphere.In one specific embodiment, the invention involves the catalyticcracking of sulfur-containing hydrocarbon feedstocks in a manner toeffect a reduction in the amount of sulfur oxides emitted from theregeneration zone of a hydrocarbon catalytic cracking unit.

Typically, catalytic cracking of hydrocarbons takes place in a reactionzone at hydrocarbon cracking conditions to produce at least onehydrocarbon product and to cause carbonaceous material (coke) to bedeposited on the catalyst. Additionally, some sulfur, originally presentin the feed hydrocarbons, may also be deposited, e.g., as a component ofthe coke, on the catalyst. It has been reported that approximately 50%of the feed sulfur is converted to H₂ S in the FCC reactor, 4% remainsin the liquid products and about 4 to 10% is deposited on the catalyst.These amounts vary with the type of feed, rate of hydrocarbon recycle,steam stripping rate, the type of catalyst, reactor temperature, etc.

Sulfur-containing coke deposits tend to deactivate cracking catalyst.Cracking catalyst is advantageously continuously regenerated, bycombustion with oxygen-containing gas in a regeneration zone, to lowcoke levels, typically below about 0.4% by weight, to performsatisfactorily when it is recycled to the reactor. In the regenerationzone, at least a portion of sulfur, along with carbon and hydrogen,which is deposited on the catalyst, is oxidized and leaves in the formof sulfur oxides (SO₂ and SO₃, hereinafter referred to as "SOx") alongwith substantial amounts of CO, CO₂ and H₂ O.

Considerable recent research effort has been directed to the reductionto sulfur oxide emissions from the regeneration zones of hydrocarboncatalytic cracking units. One technique involved circulating one or moremetal oxides capable of associating with oxides of sulfur with thecracking catalyst inventory in the regeneration zone. When the particlescontaining associated oxides of sulfur are circulated to the reducingatmosphere of the cracking zone, the associated sulfur compounds arereleased as gaseous sulfur-bearing material such as hydrogen sulfidewhich is discharged with the products from the cracking zone and are ina form which can be readily handled in a typical facility, e.g.,petroleum refinery. The metal reactant is regenerated to an active form,and is capable of further associating with the sulfur oxides when cycledto the regeneration zone.

Incorporation of Group II metal oxides on particles of cracking catalystin such a process has been proposed (U.S. Pat. No. 3,835,031 toBertolacini). In a related process described in U.S. Pat. No. 4,071,436to Blanton, et al., discrete fluidizable alumina-containing particlesare circulated through the cracking and regenerator zones along withphysically separate particles of the active zeolitic cracking catalyst.The alumina particles pick up oxides of sulfur in the regenerator,forming at least one solid compound, including both sulfur and aluminumatoms. The sulfur atoms are released as volatiles, including hydrogensulfide, in the cracking unit. U.S. Pat. No. 4,071,436 further disclosesthat 0.1 to 10 weight percent MgO and/or 0.1 to 5 weight percent Cr₂ O₃are preferably present in the alumina-containing particles. Chromium isused to promote coke burnoff.

A metallic component, either incorporated into catalyst particles orpresent on any of a variety of "inert" supports, is exposed alternatelyto the oxidizing atmosphere of the regeneration zone of an FCCU and thereducing atmosphere of the cracking zone to reduce sulfur oxideemissions from regenerator gases in accordance with the teachings ofU.S. Pat. Nos. 4,153,534 and 4,153,535 to Vasalos and Vasalos, et al.,respectively. In Vasalos, et al., a metallic oxidation promoter such asplatinum is also present when carbon monoxide emissions are to bereduced. These patents disclose nineteen different metallic components,including materials as diverse as alkaline earths, sodium, heavy metalsand rare earth, as being suitable reactants for reducing emissions ofoxides of sulfur. The metallic reactants that are especially preferredare sodium, magnesium, manganese and copper. When used as the carrierfor the metallic reactant, the supports that are used preferably have asurface area at least 50 square meters per gram. Examples of allegedly"inert" supports are silica, alumina and silica-alumina. The Vasalos andVasalos, et al., patents further disclose that when certain metallicreactants (exemplified by oxides of iron, manganese or cerium) areemployed to capture oxides of sulfur, such metallic components can be inthe form of a finely divided fluidizable powder.

Similarly, a vast number of sorbents have been proposed fordesulfurization of non-FCCU flue gases in zones outside the unit inwhich SOx is generated. In some such non-FCCU applications, the sorbentsare regenerated in environments appreciably richer in hydrogen than thecracking zone of an FCC unit. Cerium oxide is one of fifteen adsorbentsdisclosed for flue gas desulfurization in a publication of Lowell, etal., "SELECTION OF METAL OXIDES FOR REMOVING SOx FROM FLUE GAS," Ind.Eng. Chemical Process Design Development, Vol. 10, Nov. 3, 1971. In U.S.Pat. No. 4,001,374 to Longo, cerium on an alumina support is used toabsorb SO₂ from non-FCCU flue gas streams or automobile exhaust attemperatures of 572° to 1472° F., preferably 932° to 1100° F. Thesorbent is then regenerated in a separate unit by contacting it withhydrogen mixed with steam at 932° to 1472° F. During regeneration thedesorbed species is initially SO₂ and H₂ S along with excess reducinggases which can be used as feedstock for a Claus unit. The Longo patentis not concerned with reducing emissions from an FCC unit and thereducing emissions from an FCC unit and the reducing atmosphere employedin practice of this process differs significantly from thehydrocarbon-rich atmosphere in a catalytic cracker. Thus a hydrocarboncracking reaction zone is preferably operated in the substantial absenceof added hydrogen while the presence of sweeping amounts of hydrogen gasis essential to the regeneration step in practice of the process ofLongo.

D. W. Deberry, et al., "RATES OF REACTION OF SO₂ WITH METAL OXIDES,"Canadian Journal of Chemical Engineering, 49, 781 (1971) reports thatcerium oxide was found to form sulfates more rapidly than most of theother oxides tested. The temperatures used, however, were below 900° F.and thus below those preferred for use in catalyst regenerators in FCCunits.

Many commercial zeolitic FCC catalyst contain up to 4% rare earth oxide,the rare earth being used to stabilize the zeolite and provide increasedactivity. See, for example, U.S. Pat. No. 3,930,987 to Grand. The rareearths are most often used as mixtures of La₂ O₃, CeO₂, Pr₂ O₁₁, Nd₂ O₃and others. Some catalyst is produced by using a lanthanum-rich mixtureobtained by removing substantial cerium from the mixture of rare earth,.It has been found that the mere presence of rare earth in a zeoliticcracking catalyst will not necessarily reduce SOx emissions to anappreciable extent.

In accordance with the teachings of U.S. Pat. No. 3,823,092 to Gladrow,certain zeolitic catalyst compositions capable of being regenerated at arate appreciably faster than prior art rare earth exchanged zeoliticcatalyst compositions are produced by treating a previously rare earthexchanged zeolitic catalyst composition with a dilute solutioncontaining cerium cations (or a mixture of rare earths rich in cerium).The final catalysts contain 0.5 to 4% cerium cations which areintroduced to previously rare earth exchanged zeolitic catalystparticles prior to final filtering, rinsing and calcining. Cerium isdescribed as an "oxidation promoter". There is not recognition orappreciation in the patent of the effect of the cerium impregnation ofSOx stack emissions. Such impregnation of rare earth exchanged zeoliticcatalyst particles is not always effective in producing modifiedcatalysts having significant ability to bind oxides of sulfur in a FCCregenerator and release them in a FCC cracking reaction zone.

Thus, considerable amount of study and research effort has been directedto reducing oxide of sulfur emissions from various gaseous streams,including those from the stacks of the regenerators of FCC units.However, the results leave much to be desired. Many metallic compoundshave been proposed as materials to pick up oxides of sulfur in FCC units(and other desulfurization applications) and a variety of supports,including particles of cracking catalysts and "inerts", have beensuggested as carriers for active metallic reactants. Many of theproposed metallic reactants lose effectiveness when subjected torepeated cycling. Thus, when Group II metal oxides are impregnated onFCC catalysts or various supports, the activity of the Group II metalsis rapidly reduced under the influence of the cyclic conditions.Discrete alumina particles, when combined with silica-containingcatalyst particles and subjected to steam at elevated temperatures,e.g., those present in FCC unit regenerators, are of limitedeffectiveness in reducing SOx emissions. Incorporation of sufficientchromium on an alumina support to improve SOx sorption results inundesirably increased coke and gas production.

Commonly assigned U.S. patent application, namely, U.S. application Ser.No. 301,678, filed Sept. 14, 1981, now abandoned, and U.S. applicationSer. No. 301,676, filed Sept. 14, 1981, now abandoned, relate toimproved materials for reducing SOx emissions, incorporating,respectively, spinel compositions, preferably alkaline earthmetal-containing spinels, and spinel compositions including at least oneadditional metal component. The specification of each of these patentapplications is incorporated herein by reference.

Various methods have been described for the preparation of alkalineearth aluminate spinels, and particularly of magnesium aluminatespinels. According to the method disclosed in U.S. Pat. No. 2,992,191,the spinel can be formd by reacting, in an aqueous medium, awater-soluble magnesium inorganic salt and a water-soluble aluminum saltin which the aluminum is present in the anion. This patent does notteach controlling pH during the time the two salts are combined.

Another process for producing magnesium aluminate spinel is set forth inU.S. Pat. No. 3,791,992. This process includes adding a highly basicsolution of an alkali metal aluminate to a solution of a soluble salt ofmagnesium with no control of pH during the addition, separating andwashing the resulting precipitate; exchanging the washed precipitatewith a solution of an ammonium compound to decrease the alkali metalcontent; followed by washing, drying, forming and calcination steps.

There remains a need for improved spinel catalyst components, exhibitinggood SOx removal properties, and for improved processing in theirmanufacture.

SUMMARY OF THE INVENTION

This invention relates to a novel process for the improved production ofalkaline earth metal and aluminum-containing spinel compositions. Suchspinels find particular use in diminishing the emissions of sulfuroxides from combustion zones, and more particularly in conjunction withcatalytic compositions employed in hydrocarbon cracking processes.

The process of this invention further provides for the association ofone or more additional components with the alkaline earth metal,aluminum-containing spinel composition.

The improved process of this invention particularly provides for theadmixture of components in a concerted manner, whereby controlled pHconditions are maintained, and for a calcination of the resultingprecipitate conducted at a temperature capable of effective spinelformation, preferably such that a suitably high surface area isachieved.

Other objects and advantges of this invention will be apparent from thefollowing detailed description.

DESCRIPTION OF THE INVENTION

This invention broadly relates a novel process for the production ofalkaline earth metal, aluminum-containing spinel compositionscomprising:

(a) combining (a) an acidic aqueous solution containing at least onealkaline earth metal component and (b) a basic aqueous solutioncontaining at least one aluminum component in which the aluminum ispresent as an anion into an aqueous medium to form a combined massincluding a liquid phase and an alkaline earth metal,aluminum-containing precipitate, provided that the pH of the liquidphase during the combining is maintained in the range of about 7.0 toabout 9.5, preferably in the range of about 7.0 to about 8.5; and

(b) calcining the precipitate to form an alkaline earth metal,aluminum-containing spinel composition.

In a preferred embodiment, the above-noted step (a) comprisessubstantially simultaneously adding the acidic aqueous solution and thebasic aqueous solution to an aqueous liquid. In another preferredembodiment, the pH of the liquid phase is maintained in the range ofabout 7.0 to about 7.5 until no further acidic aqueous solution is to becombined.

The presently prepared spinel compositions may be used, for example, inthe form of particles of any suitable shape and size. Such particles maybe formed by conventional techniques, such as spray drying, pilling,tabletting, extrusion , bead formation (e.g., conventional oil dropmethod) and the like. When spinel-containing particles are to be used ina fluid catalytic cracking unit, it is preferred that a major amount byweight of the spinel-containing particles have diameters in the range ofabout 10 microns to about 250 microns, more preferably about 20 micronsto about 125 microns.

This invention further relates to the production of an alkaline earthmetal and aluminum-containing spinel composition which also includes atleast one additional metal component in an amount effective to promotethe oxidation of SO₂ to SO₃ at SO₂ oxidation conditions. The additionalmetal component may be added to the alkaline earth metal,aluminum-containing precipitate or spinel composition using techniques,such as impregnation, which are conventional and well known in the art.

The spinel structure is based on a cubic close-packed array of oxideions. Typically, the crystallo-graphic unit cell of the spinel structurecontains 32 oxygen atoms. With regard to magnesium aluminate spinel,there often are eight Mg atoms and sixteen Al atoms to place in a unitcell (8MgAl₂ O₄). Other alkaline earth metal ions, such as calcium,strontium, barium and mixtures thereof, may replace all or a part of themagnesium ions. Other trivalent metal ions, such as iron, chromium,gallium, boron, cobalt and mixture thereof, may replace a portion of thealuminum ions.

The presently useful alkaline earth metal and aluminum containingspinels include a first metal (alkaline earth metal) and aluminum as thesecond metal having a valence higher than the valence of the firstmetal. The atomic ratio of the first metal to the second metal in anygiven alkaline earth metal and aluminum containing spinel need not beconsistent with the classical stoichiometric formula for such spinel. Inone embodiment, the atomic ratio of the alkaline earth metal to aluminumin the spinels of the present invention is at least about 0.17 andpreferably at least about 0.25. It is preferred that the atomic ratio ofalkaline earth metal to aluminum in the spinel be in the range of about0.17 to about 1, more preferably about 0.25 to about 0.75, and stillmore preferably about 0.35 to about 0.65.

The preferred spinel composition of the present invention is magnesiumand aluminum-containing spinel composition.

The alkaline earth metal components useful in the present inventioninclude those which are suitable to provide the above-noted spinelcompositions. It is preferred that the alkaline earth metal component orcomponents employed be substantially soluble in the acidic aqueousmedium used. Examples of suitable alkaline earth metal component includenitrates, sulfates, formates, acetates, acetylacetonates, phosphates,halides, carbonates, sulfonates, oxalates, and the like. The alkalineearth metals include beryllium, magnesium, calcium, strontium, andbarium. The preferred alkaline earth metal components for use in thepresent invention are those comprising magnesium.

As noted above, the aluminum components present in the basic solutionuseful in the present invention are those in which the aluminum ispresent as an anion. Preferably, the aluminum salt is present as analuminate salt, more preferably as an alkali metal aluminate.

Any suitable acid or combination of acids may be employed in thepresently useful acidic aqueous solutions. Examples of such acidsinclude nitric acid, sulfuric acid, hydrochloric acid, acetic acid andmixtures thereof, with nitric acid, sulfuric acid and mixtures thereofbeing preferred. Any suitable basic material or combination of suchmaterials may be employed in the presently useful basic aqueoussolutions. Examples of such basic material include alkali metalhydroxides, ammonium hydroxide and mixtures thereof, with alkali metalhydroxides, and in particular sodium hydroxide, being preferred for use.The relative amounts of acids and basic materials employed are suitableto provide the desired alkaline earth metal, aluminum-containingprecipitate and the pH control as noted above.

Spinel compositions resulting from the present invention have improvedproperties relative to spinels produced without the present pH control.For example, the presently preferred spinel compositions have improvedcapabilities, e.g., stability, of reducing sulfur oxide atmosphericemissions from hydrocarbon catalytic cracking operations.

In certain embodiments of this invention, particulate materialcomprising the alkaline earth metal and aluminum-containing spinelcomposition also contains at least one additional metal component. Theseadditional metal components are defined as being capable of promotingthe oxidation of sulfur dioxide to sulfur trioxide at combustionconditions, e.g., the conditions present in a hydrocarbon catalyticcracking unit regenerator. Increased carbon monoxide oxidation may alsobe obtained by including the additional metal components. Suchadditional metal components are selected from the group consisting ofGroup IB, IIB, IVB, VIA, VIB, VIIA and VIII of the Periodic Table, therare earth metals, vanadium, iron, tin, and antimony and mixturesthereof and may be incorporated into the presently useful spinelcompositions by one or more embodiments of the process of thisinvention. The preferred additional metal component for use is selectedfrom the group consisting of bismuth, rare earth metals, chromium,copper, iron, manganese, vanadium, tin and mixtures thereof.

Generally, the amount of the additional metal component or componentspresent in the final product is small compared to the quantity of thespinel. Preferably, the final product comprises a minor amount by weightof at least one additional metal, component more preferably up to about20% by weight (calculated as elemental metal). Of course, the amount ofadditional metal used will depend, for example, of the degree of sulfurdioxide oxidation desired and the effectiveness of the additional metalcomponent to promote such oxidation. When, as is more preferred, theadditional metal component is rare earth metal component (still morepreferably cerium component), the preferred amount of this additionalmetal component is within the range of about 1 to about 20 wt. %, morepreferably about 5 to about 20 wt. % (calculated as the rare earth metaloxide) of the total final product.

The additional metal component may exist in the final product at leastin part as a compound such as an oxide, sulfide, halide and the like, orin the elemental state.

The precipitate, which is preferably dried, is calcined to yield thealkaline earth metal, aluminum-containing spinel composition. Drying andcalcination may take place simultaneously. However, it is preferred thatthe drying take place at a temperature below that which water ofhydration is removed from the spinel precursor, i.e., precipitate. Thus,this drying may occur in flowing air at temperatures below about 500°F., preferably in the range of about 150° F. to about 450° F., morepreferably about 230° F. to about 450° F. Alternatively, the precipitatecan be spray dried.

The drying of the precipitate can be accomplished in various manners,for example, by spray drying, drum drying, flash drying, tunnel dryingand the like. The drying temperature or temperatures is selected toremove at least a portion of the liquid phase. Drying times are notcritical to the present invention and may be selected over a relativelywide range sufficient to provide the desired dried product. Drying timesin the range of about 0.2 hours to about 24 hours or more may beadvantageously employed.

Spray drying equipment which is conventionally used to produce catalystparticles suitable for use in fluidized bed reactors may be utilized inthe practice of the present invention. For example, this equipment mayinvolve at least one restriction or high pressure nozzle having adiameter in the range from about 0.01 in. to about 0.2 in., preferablyfrom about 0.013 in. to about 0.15 in. The pressure upstream of thishigh pressure nozzle may range from about 400 psig. to about 10,000psig., preferably from about 400 psig. to about 7,000 psig. The materialto be dried is sent through the nozzle system into a space or chamber.The pressure in the space or chamber downstream from the nozzle systemis lower than that immediately upstream of the nozzle and is typicallyin the range from about 0 psig. to about 100 psig., preferably fromabout 0 psig. to about 20 psig. Once through the nozzle, the material tobe dried is contacted for a relatively short time, e.g., from about 0.1seconds to about 20 seconds with a gas stream which is at a temperatureof from about 200° F. to about 1500° F., preferably from about 200° F.to about 750° F. The gas stream which may be, for example, air or theflue gases from an inline burner (used to provide a gas stream havingthe proper temperature) or a substantially oxygen-free gas, may flowco-current, counter-current or a combination of the two relative to thedirection of flow of the material to be dried. The spray dryingconditions, such as temperatures, pressures and the like, may beadjusted because, for example, of varying the composition of thematerial to be dried to obtain optimum results. However, thisoptimization may be achieved through routine experimentation.

An alternative to the high pressure nozzel described above is the"two-fluid" nozzle in which the material to be dried is dispersed by astream of gas, typically air. The two fluid nozzle has the advantage oflow operating pressure, e.g., from about 0 psig. to about 60 psig. forthe material to be dried and from about 10 psig. to about 100 psig. forthe dispersing gas. The dispersing gas may also function as at least aportion of the drying gas stream. The various operating parameters notedabove may be varied in order to achieve the correct or desired boundparticle size.

In order to minimize contact between the chamber walls and wet material,the chamber downstream from the nozzle system is large in size, e.g.,from about 4 to about 30 feet in diameter and from about 7 to about 30feet long, often with an additional conical shaped portion forconvenient withdrawal of the dried material. The spray drying apparatusmay also include separation means, e.g., cyclone separators, in theoutlet gas line to recover at least a portion of the dried materialentrained in this stream.

Suitable calcination temperatures for the precipitate are in the rangeof about 1000° F. to about 1800° F. However, it has been found thatimproved spinel formation occurs when the calcination temperature ismaintained within the range of about 1050° F. to about 1600° F., morepreferably about 1100° F. to about 1400° F. and still more preferablyabout 1150° F. to about 1350° F. Calcination of the precipitate may takeplace in a period of time in the range of about 0.5 hours to about 24hours or more, preferably in a period of time in the range of about 1hour to about 10 hours. The calcination of the precipitate may occur atany suitable conditions, e.g., inert, reducing or oxidizing conditions,which oxidizing conditions be preferred.

In one embodiment of the process of this invention it has beendiscovered that improved spinel compositions are afforded byimpregnation procedures. Such preparative procedures preferably comprisethe impregnation of at least one or certain additional metal components,noted previously, on the precipitate or the spinel composition, followedby drying and, preferably, calcination.

In one preferred embodiment of this invention, calcination of the spinelcomposition after contacting with the additional metal component orcomponents is effected at oxidizing conditions, e.g., in a stream offlowing air. These conditions are especially preferred when a ceriumcomponent is present in the formulation in order to prevent or minimizeinteraction between cerous ions and the spinel base.

A preferred alkali metal aluminate is sodium aluminate. Although themineral acid may be nitric, hydrochloric, or sulfuric acid, tocorrespond to the selected alkaline earth metal salt, care must be takento employ water-soluble salts and, accordingly, the preferred alkalineearth metal salt is magnesium nitrate and the preferred mineral acid isnitric acid.

The concerted technique of this invention affords a precipitate phasewhich may be directly washed with water or, optionally, first permittedto age for up to about 24 hours at ambient temperature or elevatedtemperatures, prior to any further processing. Separation of theprecipitate phase may be accomplished by any conventional means, such asfiltration.

The products prepared by the process of this invention exhibit superiorproperties as sulfur oxide reduction materials, e.g., in fluid catalystcracking operations, when compared with similar products prepared byconventional methods. For example, the products of this invention havesuitable mechanical strength and bulk density, low attrition rate,suitable surface area and pore volume, and good fluidizationcharacteristics.

The process of this invention provides spinel compositions exhibitingsurface areas ranging from about 25 to about 600 m.² /g.

The embodiments described below are exemplary, without limitation, ofthe process of this invention.

EXAMPLE I

7.05 lb. sodium aluminate (analyzed as 29.8% by weight Na₂ O and 44.85%by weight of Al₂ O₃) was stirred with one gallon deionized water tobring as much as possible into solution. This was filtered through clothwith a 10" Buchner funnel. The filtered solution was diluted to 8 literswith deionized water.

7.95 lb. Mg(NO₃)₂ 6H₂ O was dissolved in one gallon dionized water, and166 ml. of concentrated HNO₃ was added. The solution was diluted to 8liters with deionized water.

The two final solutions were run simultaneously from burettes into 32liters dionized water in a 30 gallon rubber lined drum. The mix wasstirred vigorously during the addition. Addition of the Mg(NO₃)₂solution required 36 minutes. 2760 ml. of the sodium aluminate solutionwas added during this period. The pH was held between 7.0 and 7.5. Afteraddition of all the magnesium nitrate-containing solution, sodiumaluminate solution was added to bring the pH to 8.5. After this, 1080ml. of sodium aluminate solution remained and was discarded.

The mix was held overnight and then filtered with a plate-frame press.The cake was washed in the press with 110 gallons deionized water. Asolution of 26 grams Mg(NO₃)₂ 6H₂ O in 200 ml. deionized water was addedto the slurry. The slurry was filtered and washed as before. After arepeat of the slurry, filter, and wash, the cake was dried at about 250°F. in a forced air drying oven.

The dried product was then hammermilled, first on a 0.050" screen, thenthe 0-60 mesh portion was hammermilled again, this time on the 0.010"screen. The desirable, fine material was then screened through a 60 meshscreen. The so-obtained product, magnesium aluminate spinel precursor,was then transferred into a 59 mm. diameter quartz tube, where it wascalcined, in a fluidized state, for 3 hours at 900° F. with an air flowrate of about 106 liters per hour to form magnesium aluminate spinelwhich was found to have an atomic ratio of magnesium to aluminum ofabout 0.48.

For cerium impregnation, 0.39 lb. cerium carbonate was slurried in 1820mls. of water and mixed with 350 mls. of 70% nitric acid slowly todissolve the carbonate. 3.75 lbs. of the calcined magnesium aluminatespinel was placed in a Pyrex tray and impregnated with the ceriumsolution dissolving and hand mixing using rubber gloves. After theimpregnation was complete, the mix was allowed to equilibrate overnight.

The ipregnated product was dried under IR lamps and finally in a 260° F.oven overnight. The dried product was calcined in a fluidized state in a59 mm. diameter quartz reactor, for 3 hours at 900° F. with an air flowrate of about 83 l/hr. The resulting magnesium aluminate spinelparticles were screened to produce final particles having diameters lessthan 100 microns and these final particles contained 5% by weight ofcerium, calculated as elemental cerium.

EXAMPLE II

Example I is repeated except that final magnesium aluminate spinelparticles were impregnated, using conventional techniques, with anaqueous solution of chloroplatinic acid. The resulting particles aredried and calcined and contain about 5% by weight of cerium, calculatedas elemental cerium and about 100 ppm. of platinum, by weight of thetotal platinum-containing particles, calculated as elemental platinum.The cerium and platinum are substantially uniformly distributed on thespinel-containing particles.

EXAMPLE III

An aqueous solution of magnesium nitrate was prepared by dissolving179.5 g. (1.21 moles) of crystalline magnesium nitrate in deionizedwater, followed by the addition thereto of sufficient concentratednitric acid to provide 34.0 (0.54 mole) HNO₃

An aqueous solution of sodium aluminate was prepared by dissolving 164g. (1.0 mole) sodium aluminate (Na₂ Al₂ O₄) and 22.4 g. (0.56 mole)sodium hydroxide in 800 g. deionized water.

The solutions of magnesium nitrate and sodium aluminate were addedsimultaneously, with stirring, to a heel of 2000 g. deionized water atrespective rates set to maintain the pH of the mixture between 7.0 and7.5. Upon completion of the addition of the magnesium nitrate solution,additional sodium aluminate solution was added until the pH of themixture reached 8.5. The precipitate phase was allowed to stand for 24hours, filtered, slurried with water and refiltered twice, and finallydried for 3 hours at 260° F. in a stream of flowing air.

The dried filter cake was ground in a hammermill until the fine materialpassed through a 60-mesh screen. The ground material was then calcinedin a stream of flowing air for 3 hours at 1350° F., to produce amagnesium, aluminum-containing spinel composition.

EXAMPLE IV

The spinel compositions prepared as in Examples I-III are tested forsulfur pick-up capabilities as follows. Each of these materials isfluidized in a gas stream, comprising (by volume) 5% O₂, 10% SO₂ and 85%N₂, after heating at 1100° F. in a stream of nitrogen gas. After a30-minute treatment with the SO₂ -containing gas, remaining SO₂ isflushed out with nitrogen. After cooling, analyses for sulfur areconducted on the solids and on the gas stream to determine theefficiency of SOx pickup by formation of metal sulfates. Each of thesematerials is found to have a substantial capability to pick-up sulfur.For example, the spinel composition of Example III picked up 51% of thesulfur dioxide.

EXAMPLE V

The sulfur-containing spinel compositions from Example IV are heated to1000° F. in flowing nitrogen gas and then for 30 minutes in a stream ofhydrogen. Each spinel composition is flushed with nitrogen, and, aftercooling, is analyzed for sulfur content, to determine the efficiency ofsulfur removal by reduction of metal sulfates. Each of these materialsis found to have a substantial capability to release sulfur under theconditions of the above-noted treatment. For example, the spinelcomposition of Example III released 63% of the sulfur.

While this invention has been described with respect to various specificexamples and embodiments, it is to be understood that the invention isnot limited thereto and that it can be variously practiced within thescope of the following claims:

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A process for theproduction of an alkaline earth metal, aluminum-containing spinelcomposition comprising:(a) combining (a) an acidic aqueous solutioncontaining at least one alkaline earth metal component and (b) a basicaqueous solution containing at least one aluminum component in which thealuminum is present as an anion into an aqueous medium to form acombined mass including a liquid phase and an alkaline earth metal,aluminum-containing precipitate, provided that the pH of said liquidphase during said combining is maintained in the range of about 7.0 toabout 8.5 (b) calcining said precipitate to form said alkaline earthmetal, aluminum-containing spinel composition.
 2. The process of claim 1wherein step (a) comprises substantially simultaneously adding saidacidic aqueous solution and said basic aqueous solution to an aqueousliquid.
 3. The process of claim 1 wherein the atomic ratio of alkalineearth metal to aluminum in said spinel composition is in the range ofabout 0.17 to about
 1. 4. The process of claim 1 wherein the atomicratio of alkaline earth metal to aluminum in said spinel composition isin the range of about 0.25 to about 0.75.
 5. The process of claim 1wherein the atomic ratio of alkaline earth metal to aluminum in saidspinel composition is in the range of about 0.35 to about 0.65.
 6. Theprocess of claim 1 wherein said calcining takes place at a temperaturein the range of about 1000° F. to about 1800° F.
 7. The process of claim1 wherein said calcining takes place at a temperature in the range ofabout 1050° F. to about 1600° F.
 8. The process of claim 1 wherein saidcalcining takes place at a temperature in the range of about 1100° F. toabout 1400° F.
 9. The process of claim 1 wherein said precipitate isdried to remove at least a portion of said liquid phase prior to beingcalcined.
 10. The process of claim 1 wherein said precipitate or saidspinel composition is impregnated with at least one additional metalcomponent to provide a spinel composition which includes a minor amountof at least one additional metal component effective to promote theoxidation of SO₂ to SO₃ at SO₂ oxidation conditions.
 11. The process ofclaim 9 wherein said precipitate is dried at a temperature of less thanabout 500° F.
 12. The process of claim 10 wherein said additional metalis selected from the group consisting of bismuth, rare earth metals,chromium, copper, iron, manganese, vanadium, tin and mixtures thereof.13. The process of claim 1 wherein said aluminum component is alkalimetal aluminate.
 14. The process of claim 1 wherein the pH of saidliquid phase is maintained in the range of about 7.0 to about 7.5 untilno further acidic aqueous solution is to be combined.
 15. The process ofclaim 9 wherein said precipitate is spray dried and said spinelcomposition is in the form of particles having diameters in the range ofabout 10 to about 250 microns.
 16. The process of claim 9 wherein saidprecipitate is spray dried and said spinel composition is in the form ofparticles having diameters in the range of about 20 to 125 microns. 17.The process of claim 9 wherein said precipitate is maintained in contactwith at least a portion of said liquid phase for a period of up to about24 hours before being dried.
 18. The process of claim 10 wherein saidspinel composition includes up to about 20% by weight, calculated aselemental metal, of at least one of said additional metal component. 19.The process of claim 12 wherein said spinel composition includes up toabout 20% by weight, calculated as elemental metal, of a least one ofsaid additional metal component.